Today, both strength and a low kappa number, i.e. a low lignin content after cooking, are both required of chemically prepared pulp. The requirements are in conflict unless cooking conditions are carefully optimized. These cooking conditions include a correct alkali split, a suitable temperature profile, an adequate amount of liquid, and as small gradients as possible especially in the radial direction of a continuous digester. This requires high circulation and expansion flows.
Because even a partial blocking of expansion and circulation screens causes channelling and disturbed flows in the digester, and often also production losses, it is not always possible to maintain the conditions required for making strong and clean pulp. This is of course a very negative matter, because it causes an increased use of raw material, energy and chemicals, which again increases production costs and environmental loads. Sometimes, blocking also causes the screens to break when the support structures fail.
The screens presently in the market usually comprise a group of bar screens which are often arranged in a manner resembling a chessboard in a desired location on the inner wall of the digester. The screen bars of a bar screen are angular in shape, for instance metal sections machined nearly to the shape of the letter T. The top edge of the T profile then acts as the end face towards plug flow. Screens of this type are disclosed in WO 9419533, for instance. Each installed “chessboard square” comprises a group of screen bars arranged in an upright position and side by side. Between the screen bars, there is a gap through which liquid is sucked. The screen bars are fastened in as parallel manner as possible to a cross member and the screen is equipped with an angle iron frame. In every second square of the chessboard pattern, there is a solid metal plate instead of the bar screen to prevent the formation of a from-screen-to-screen-growing “doughnut” on the jacket of the digester. The doughnut in question slows down the movement of the plug flow in a continuous digester, for instance.
One problem with this known screen construction is that it has a relatively high tendency to block when chips fasten to the gaps. This is due both to the radial wood chip pressure against the screen generated by the gravitational force directed to the chip column and to the radial speed of the liquid outward through the screen. The speed is approximately five times higher than the downward speed of the chips. Typically, the speed of liquid is in the range of 10 to 15 mm/s, whereas the corresponding downward speed of chips is approximately 2 to 3 mm/s. When the compaction rate of the digester is normal, the chips bind each other in place inside the plug flow and partly also on the rim of the plug flow. Small slivers and chip particles follow the partially faster radial liquid flow and fasten to the sharp edges of the screen bars. More chip particles can then settle on such a fastened chip particle and block an even larger area of the screen. A softened chip particle is also easily cut by the sharp edges of the screen bars, which also tends to block the screen. The tendency to blockage described above becomes worse as a bigger area of the open screen surface is blocked, because this generates a bigger load and, at the same time, a bigger danger of blocking in the screen surface still remaining open.
In addition, the backgrounds of the screens are blocked, because flow rates become considerably slower in the backgrounds of the screens and the heavier particles in the liquid start to settle down in the background and finally block the flow area behind the screen.
Due to the positioning of the screen and the manner of blockage, it is very difficult to clean the screen and this should, thus, be avoided at all costs.
Kvaerner Pulping Technologies AB have in their patent SE-B-501243 proposed a horizontal slotted screen as a solution to the blockage problem, the screen being characterized in that the screen bars are horizontal. The screen bars are square or diamond-shaped in cross-section, i.e. sharp cornered. The critical characteristic of this solution is the fact that chip particles lie flat on top of each other like playing cards in the plug flow, and thus, when moving radially along with the lye flow, they easily wedge in the horizontal gaps of the screen or fasten to the sharp corners of the screen bars.
Another and more critical issue is slivers. There is always a large number of slivers among chips. These slivers are approximately the size of matches, sometimes bigger, sometimes smaller. Their diameter can be 1 to 3 mm. A part of these slivers are found sooner or later in the plug flow at the wall of the digester and the screens while the plug flow flows on. A part of the slivers are taken along by the flowing alkaline cooking liquor and they fasten to the sharp corners of the screen bars and/or to other chip particles already fastened to the gaps, thus forming a new fastening surface.
The background volume of the screens is relatively large and causes a decrease in the flow rate and thus deposits.
In other words, prior art screens are difficult to run and many people find that the operation of the bar screens is the Achilles heel of KAMYR digesters.